Transcript The TELNET Protocol - Bambang Nurcahyo Prastowo

The TELNET Protocol
Reference: RFC 854
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TELNET vs. telnet

TELNET is a protocol that provides “a
general, bi-directional, eight-bit byte
oriented communications facility”.

telnet is a program that supports the
TELNET protocol over TCP.

Many application protocols are built
upon the TELNET protocol.
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The TELNET Protocol
TCP connection
 data and control over the same
connection.
 Network Virtual Terminal
 negotiated options

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Network Virtual Terminal
intermediate representation of a generic
terminal.
 provides a standard language for
communication of terminal control
functions.

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Network Virtual Terminal
Server
Process
NVT
NVT
TCP
TCP
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Negotiated Options
All NVTs support a minimal set of
capabilities.
 Some terminals have more capabilities
than the minimal set.
 The 2 endpoints negotiate a set of
mutually acceptable options (character
set, echo mode, etc).

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Negotiated Options
The protocol for requesting optional
features is well defined and includes
rules for eliminating possible negotiation
“loops”.
 The set of options is not part of the
TELNET protocol, so that new terminal
features can be incorporated without
changing the TELNET protocol.

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Option examples

Line mode vs. character mode

echo modes

character set (EBCDIC vs. ASCII)
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Control Functions

TELNET includes support for a series of
control functions commonly supported
by servers.

This provides a uniform mechanism for
communication of (the supported)
control functions.
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Control Functions

Interrupt Process (IP)
– suspend/abort process.

Abort Output (AO)
– process can complete, but send no more
output to user’s terminal.

Are You There (AYT)
– check to see if system is still running.
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More Control Functions

Erase Character (EC)
– delete last character sent
– typically used to edit keyboard input.

Erase Line (EL)
– delete all input in current line.
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Command Structure
All TELNET commands and data flow
through the same TCP connection.
 Commands start with a special
character called the Interpret as
Command escape character (IAC).
 The IAC code is 255.
 If a 255 is sent as data - it must be
followed by another 255.

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Looking for Commands
Each receiver must look at each byte
that arrives and look for IAC.
 If IAC is found and the next byte is IAC a single byte is presented to the
application/terminal (a 255).
 If IAC is followed by any other code the TELNET layer interprets this as a
command.

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Command Codes
IP
 AO
 AYT
 EC
 EL

243
244
245
246
247
WILL
 WON’T
 DO
 DON’T
 IAC

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252
253
254
255
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Playing with TELNET
You can use the telnet program to
play with the TELNET protocol.
 telnet is a generic TCP client.

– Sends whatever you type to the TCP
socket.
– Prints whatever comes back through the
TCP socket.
– Useful for testing TCP servers (ASCII
based protocols).
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Some TCP Servers you can
play with

Many Unix systems have these servers
running (by default):
– echo
– discard
– daytime
– chargen
port 7
port 9
port 13
port 19
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telnet hostname port
> telnet rcs.rpi.edu 7
Trying 128.113.113.33...
Connected to cortez.sss.rpi.edu
(128.113.113.33).
Escape character is '^]'.
Hi dave
Hi dave
stop it
stop it
^]
telnet> quit
Connection closed.
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telnet vs. TCP

Not all TCP servers talk TELNET (most don't)

You can use the telnet program to play with
these servers, but the fancy commands won't
do anything.
– type ^], then "help" for a list of fancy TELNET stuff
you can do in telnet.
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